1. Field of the Invention
This invention relates to an automated dry deposition sampler for direct measurement of downward or upward flux of materials in the atmosphere during periods without precipitation.
2. Description of Related Art
There are two fundamentally different types of sampling of airborne materials: 1. sampling of concentrations of particles or chemical compounds in the air which measure in units of mass per volume; and 2. sampling of flux of particles or chemical compounds in the air which measurements have units of mass per area per time. The dry deposition sampler of this invention falls into the latter class.
A number of patents relate to samplers for measurement of airborne concentrations of particles: U.S. Pat. Nos. 3,914,979; 3,479,869 and 3,681,973 teach sampling devices having a wind vane to keep the device headed into the wind; slotted particle impactors are taught by U.S. Pat. No. 4,764,186 and multiple nozzle impactors are taught by U.S. Pat. No. 4,133,202; U.S. Pat. No. 3,620,078 teaches a roto-slide particle sampler having automatically opening shields to protect collection plates from wind compaction; U.S. Pat. No. 4,827,779 teaches a personal impactor having a porous impactor plate cartridge with a lightly oiled surface through which air is drawn by a pump; U.S. Pat. No. 2,973,642 teaches a rotatable sampler having a tacky adhesive surface; and U.S. Pat. No. 3,633,405 teaches a multistage rotary inertial impaction sampler.
U.S. Pat. Nos. 4,774,836 and 4,932,254 teach detectors for dry deposition measurement of flux of airborne materials. These patents teach a radially symmetrical airfoil-type surface having a flat circular surrogate sample collection surface mounted centrally on the top surface, the airfoil surface providing a laminar or transition boundary layer over the surrogate surface. The surrogate sampling surface may be coated with coating capable of retaining the contaminants. The structure forming the airfoil-type surface is supported by a post extending from the center of its bottom surface. The dry deposition detectors taught by these patents are limited to measurement of downward flux and cannot be used to measure upward flux, as may be desired in many cases. A further disadvantage of the dry deposition detectors taught by these patents is that they must be attended, that is, they must be removed and replaced or covered and uncovered by someone with time periods noted at each occasion of rain or snow. Thus, the dry deposition detectors taught by these patents cannot be left unattended for desired long periods of atmospheric sampling.
Wind tunnel testing of small particle deposition on flat plates has been reported by D. I. McCready, Wind Tunnel Modeling of Small Particle Deposition, Aerosol Sci. Technol., Vol. 5, pp. 301-312, (1986).
Study of deposition of particles from the atmosphere to single greased sample collectors on the upper and lower side of a single flat dry deposition plate having a wind vane attached directly to the singly dry deposition plate and a horizontal shield over the dry deposition plate has been reported by K. E. Noll, Fang, K. Y. P. and Watkins, L. A., Characterization of the Deposition of Particles from the Atmosphere to a Flat Plate, Atmospheric Environment, Vol. 22, No. 7, pp. 1461-1468, (1988). A dry deposition model for atmospheric coarse particles using sample collectors described in Noll, et al, supra, has been described by K. E. Noll and Fang, K. Y. P., Development of a Dry Deposition Model for Atmospheric Coarse Particles, Atmospheric Environment, Vol. 23, No. 3, pp. 585-594, (1989).